In the United States trucks are classified according to their Gross Vehicle Weight Rating (GVWR). For example, Classes 1 through 8 covers trucks with GVWRs up to 33,000 pounds. Such trucks include a steering axle assembly mounted to a frame portion of the truck. The steering axle assembly provides a means of pivotally controlling the front wheels of the truck for steering the truck.
Typically, the steering axle assemblies includes a king pin assembly which is a pivotal connector between the wheel end and an axle beam. The axle beam is mounted to the frame and the wheel end provides a mounting area for rotatingly mounting the wheels to the wheel end. The king pin assembly also includes a knuckle which defines a pair of opposing arms extending from a body portion of the knuckle in a C-shaped configuration. A distal end of each of the arms includes a first bore extending therethrough. A connecting end of the axle beam has a second bore extending therethrough. The connecting end of the axle beam is positioned between the opposing arms such that the first and second bores are aligned about a common axis. A king pin is disposed in the first and second bores. The king pin is pinned to the connecting end of the axle beam. The wheel end and knuckle is pivotable with respect to the king pin.
Certain types of king pin assemblies include a thrust bearing mounted between one or both of the arms and an opposing surface of the connecting end of the axle beam. In addition, a bushing such as a sleeve bushing is typically positioned in the first bore with the king pin positioned in an internal area of the sleeve. Grease fittings such as Zerk fittings are typically provided in the connecting end of the axle beam and/or the arms of the knuckle to provide a path for introduction of lubricants such as grease into the thrust bearing and/or the sleeve.
The thrust bearings and sleeves can be exposed to contaminants such as dirt, sand and salt, which can degrade and wear the thrust bearings and sleeves. However, lubrication, overhaul, maintenance and replacement of king pin assemblies is time consuming, difficult and expensive. Use of seals installed on the thrust bearings and sleeves have been unsuccessful in stopping the ingress of contaminants therein and egress of lubricants therefrom.
For decades, those skilled in the relevant art have sought after a king-pin assembly that requires no maintenance for the duration of the life of the king pin assemblies, which is typically one million miles. However, such a maintenance-free king pin assembly has not been achieved. Design of king pin assemblies involves a delicate balance and tradeoffs between friction and wear resistance. Those skilled in the relevant art have found that while roller bearings have relatively low operating torque, they can have shortened life, because proper lubrication is often not maintained. When a roller bearing is properly lubricated it will have insignificant wear. However, if the roller bearing experiences high loads, it can suffer for surface fatigue, referred to as spalling. On the other hand, self-lubricated materials are known to have a high load capacity but operate at a higher torque than comparable roller bearings. In addition, self-lubricated bearing materials tend to wear with use. Thus those skilled in the relevant art have been discouraged from using self-lubricated materials in bearings for steering systems because of the potential for bearing wear which can cause steering system lashing and also driver discomfort and dissatisfaction with the steering system operation. For example, one symptom of such wear would be the wheels inappropriately returning to the center.